To clarify a bit, how is the 192.168.10.0 network currently getting over to the Ubiquiti AP's?

You can do what you are describing. Using the Sonicwall to hand out guest IP's would be the right now. You can probably (not familiar enough with Sonicwalls to be sure) just make the guest Network a sub interface (aka, vlan) off the corporate network uplink to the Sonicwall rather then each needing their own physical port. That's how I do it on my Fortigates. Been about 9 years since I've touched a Sonicwall though.

Just be sure Firewall rules are such that the guest Network can go to the internet, but NOT to the corporate network.

FWIW: If it were me, I'd change the IP range of the guest Network so it's somewhere in the 172.16.0.0/20 space. I like guest IP's to look VERY different from internal IP's so it's abundantly obvious if a internal user or guest gets setup wrong. I did one guest network as 172.25.31.0/24, and defined it as VLAN 2531 so it stands out like a sore thumb.

In the controller software, go to Settings, Network, edit the LAN network and make sure the purpose is set to Corporate, you set an IP address - 192.168.1.1 or whatever in the Gateway/Subnet box, and on the same page, make sure DHCP Mode is set to DHCP server. When you choose DHCP Server, the DHCP options should come up - set the IP address range you want to hand out - 192.168.1.100 - 192.168.1.199 or whatever, Name Server can stay at Auto if you want to use your ISP's DNS Servers or you can put in the ones you want to use, DHCP Gateway should stay at auto and you shouldn't need to change anything else.

Save and let it provision.

When it's completed, reboot your workstation and make sure it pulled an IP address from the range you set and that the default gateway is set to the address of the USG.

If you don't have Internet access at this point, use your browser and go to the IP address of the USG, log in, click Configuration and make sure the connection type is set to DHCP. (I assume since this is a home they don't have a static IP assigned).

You probably need to power cycle the modem at some point as most require you to do that when you change the device directly behind it.

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It should hand out DHCP, but I'm just naturally pre-programmed to set myself a static in the 192.168.1.0 network when working with Ubnt stuff.

Haven't messed with too many of them yet, but I haven't had any trouble plugging in their WAN/LAN links and going to the default management IP from the LAN side and doing the bare bones network setup from there.

You'll have to change the network to get it on the same subnet as the controller if you're not doing L3 adoption.

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In the controller software, go to Settings, Network, edit the LAN network and make sure the purpose is set to Corporate, you set an IP address - 192.168.1.1 or whatever in the Gateway/Subnet box, and on the same page, make sure DHCP Mode is set to DHCP server. When you choose DHCP Server, the DHCP options should come up - set the IP address range you want to hand out - 192.168.1.100 - 192.168.1.199 or whatever, Name Server can stay at Auto if you want to use your ISP's DNS Servers or you can put in the ones you want to use, DHCP Gateway should stay at auto and you shouldn't need to change anything else.

Save and let it provision.

When it's completed, reboot your workstation and make sure it pulled an IP address from the range you set and that the default gateway is set to the address of the USG.

If you don't have Internet access at this point, use your browser and go to the IP address of the USG, log in, click Configuration and make sure the connection type is set to DHCP. (I assume since this is a home they don't have a static IP assigned).

You probably need to power cycle the modem at some point as most require you to do that when you change the device directly behind it.